Recently, there have been active studies that attempt to enhance driving safety by mounting on a vehicle such as an automobile a radar system that automatically detects surrounding obstacles and the like to make use of the detection results for vehicle cruise control. The radar system used in the vehicle cruise control needs to detect relative speed information on obstacles and other vehicles with high accuracy.
While such radar system detects speeds, positions, and the like of target objects by frequency-analyzing reception signals obtained by receiving incoming radio waves from the target objects, in order to improve frequency resolution in the frequency analysis of the reception signals, it is necessary to receive the signals for more than a predetermined time period. Taking more time for measurement of the reception signals, the frequency resolution is improved accordingly, which increases accuracy of measurement values outputted as the result.
A minimum measurement time required for achieving a desired velocity resolution can be determined in principle depending on a frequency analysis method to be employed. For example, in a radar system using transmission waves of wavelength λ, if reception signals are frequency-analyzed by a Fourier transformation, a minimum measurement time Tc required for obtaining a velocity resolution δV and a frequency resolution δf is known to satisfy Formula (1):
                              δ          ⁢                                          ⁢          V                =                                            λ              2                        ⁢            δ            ⁢                                                  ⁢            f                    =                                    λ              2                        ·                                          1                Tc                            .                                                          (        1        )            
Cost reduction is a key for vehicle-mounted radars to come into widespread use. In order to achieve cost reduction of radar systems, a frequency-modulated continuous wave (FMCW) method or a two-frequency continuous wave (CW) method is likely to be advantageous that is feasible with low speed signal processing compared with a pulse radar or a pulse compression radar (spread spectrum radar) that requires a signal processing circuit of high performance.
The FMCW method is a method in which beat signals are created from transmitted signals and received signals obtained by receiving reflection waves from target objects during two measurement periods of an up-chirp and a down-chirp in frequency, to detect relative speeds of and distances to the target objects by combining beat signals obtained during the up-chirp period with those obtained during the down-chirp period. In this case, since the respective beat signals needs to be determined independently from the up-chirp and the down-chirp periods, both up-chirp and down-chirp periods have to be equal to or more than the time Tc. Accordingly, it takes at least twice the time Tc to obtain measurement values satisfying a desired velocity resolution.
On the other hand, the two-frequency CW method is a method in which transmission waves of two frequencies f1 and f2 each are emitted for a fixed time period, to detect target objects from frequency and phase information of respective received waves. In this case also, since frequency-analysis process of the received waves for the transmission waves of frequency f1 and those for the transmission waves of frequency f2 are independent of each other, it is needed that after the transmission waves of frequency f1 have been emitted at least for the time Tc and their reflection waves have been received, the transmission waves of frequency f2 are emitted at least for the time Tc and their reflection waves are received. For that reason, in order to obtain measurement values satisfying a desired velocity resolution even in the two-frequency CW method, it resultantly takes at least twice the time Tc.
Thus, either the conventional FMCW or two-frequency CW method needs a measurement time that is twice or more the time Tc in order to obtain measurement values with a desired velocity resolution. As a method of obtaining measurement values within twice the time Tc, a method is known in which relative speeds of and distances to target objects are detected by combining phase differences of reception wave beat signals obtained from transmission waves of different frequencies during either an up-chirp or a down-chirp period in an FMCW method (refer to Patent Document 1, for example).
Patent Document 1: “Method and Device for Determining Separation and Relative speed of a Distant Object”; Japanese Unexamined Patent Publication No. 2004-511783.
Patent Document 2: “Radar System”; Japanese Unexamined Patent Publication No. 2002-71793.